The invention relates generally to detection of fiber optic cable, and more specifically to detection of fiber optic cable embedded within a structure such as a wall of a building. More particularly, the invention relates to detecting a fiber optic cable within a hollow building wall by detecting an electrical field generated by movement of a charged fiber optic cable contained within the building wall. The invention is particularly useful when only one side of a wall structure is accessible and there is no access to the fiber optic cable.
Many, if not all, fiber optic cables installed within the walls of buildings are totally non-conductive, including any strength member. The absent of conductors makes it difficult to locate cables positioned within building walls using conventional techniques. Conventional devices for detection of features embedded within a wall structure may rely on one or more diverse technologies to accomplish their purpose. Most commonly used detection techniques use capacitive, magnetic, ultrasonic and electromagnetic detectors. U.S. Pat. No. 6,215,293 discloses a device that may be used for locating a wooden object, a metal object or a live AC wire positioned behind a wall surface, as the device is moved along the wall surface. The disclosed invention uses capacitive techniques for detecting a change in dielectric constant due to embedded non-metallic features, magnetic techniques for detecting a change in reluctance due to embedded metallic features, and electromagnetic techniques for detecting radiation due to alternating current flowing in a wire. A disadvantage of the approach disclosed in this invention is the near proximity requirement between the detectors and the embedded features, due to the lack of detector sensitivity and selectivity.
Finding fiber optic cables in walls of buildings is very difficult, especially when access to only one side of the wall structure is available. An investigation was conducted of technologies that may be useful for detecting fiber cables hidden in walls with access from only one side of the wall. These technologies include (1) photon-induced positron annihilation (PIPA), (2) nuclear magnetic resonance (NMR), (3) x-ray radiography (4) dielectric measurement, (5) microwave polarization, and (6) detecting an electric field generated from a moving fiber cable that contains a static charge. The study of the PIPA approach showed that it is not applicable or practical for finding hidden fiber optic cables. Testing showed that the NMR approach is also not practical. Testing of x-ray radiography techniques showed that if access to both sides of a wall structure is available, through-transmission x-ray techniques may be used to effectively see through walls made from sheetrock up to approximately 0.50 inch thick for detection of fiber optic cables greater than 0.02 inch in diameter. The drawback of x-ray radiography techniques is that it requires access to both sides of a wall, which may not be available. Evaluation of the dielectric measurement method showed that fiber cable could be detected if it was within approximately 0.38 inch from the outside surface of a wall board, embedded in the sheetrock. In most cases, fiber optic cable is suspended in a wall and not embedded in sheetrock. Evaluation of microwave polarization techniques did indicate limited capability to detect fiber optic cable within wall structures. However, this technique showed high sensitivity to the distance between the microwave probe and the fiber cable, making this approach impractical. The most practical method for identifying fiber cables within a wall structure is detecting an electric field generated by an electrically charged fiber optic cable that is exhibiting movement.